1. Field of the Invention
The present invention is directed to a method and apparatus for manufacturing optical cables and, in particular, optical cables which include flextubes having optical-fibers therein.
With the advent of local area networks and the relative broadband capabilities of fiber optic links, it has become commonplace for new communication systems to include fiber-optic capabilities. Communication cables employing optical fibersxe2x80x94optical cablesxe2x80x94are widely used in the telecommunications industry. In particular, multifiber optical cables are widely used for long distance telephone communications, interexchange telephone applications, and other telephony and data transmission applications. Optical cables are also being incorporated into cable television networks in place of more traditional coaxial cables. Optical cables may permit long distances between signal repeaters or eliminate the need for such repeaters altogether. In addition, optical fibers offer extremely wide bandwidths and low noise operation.
2. Related Art
In the use of optical fibers, optical cables are provided for physical protection of the fibers in view of the fragile nature of the glass optical fibers. An optical cable may contain many optical fibers which must be identified and manipulated without disturbing other optical fibers within the optical cable. Therefore, optical cables may have various internal structures. The structure families which are currently being used are tight tube, monotube, slotted core, and loose tube.
In the tight buffer tube construction, protective layers are applied in direct contact with each optical fiber so there is no fiber overlength. In such a tight buffered construction, each optical fiber has one or more completely encapsulating layers in order to provide mechanical protection. The protective layers may be made of thermoplastic or other suitable materials. The protective layers typically have material properties which give the buffered fiber good mechanical and thermal performance. The value of cable tensile elongation for the buffered fiber is typically less than 0.15% in order to provide low attenuation increase at low temperatures.
In the monotube structure, all of the optical fibers are housed in a single, centrally located, gel filled, oversized, thermoplastic, buffer tube. The optical fibers may be loosely configured, grouped in bundles wrapped by binders, or held in a matrix by ribbons. The hollow buffer tube is typically filled with a thixotropic gel which blocks water penetration, but allows for fiber movement during cable expansion or contraction. A precise amount of fiber overlength within the buffer tube is required in order for the fibers to maintain a virtual stress-free condition during cable expansion. The amount of overlength is typically within 0.1-0.2% of the value for the amount of cable tensile elongation.
The slotted core structure has optical fibers precisely placed in gel filled channels or slots. The channels are symmetrical and form a helical path along the longitudinal axis of the cable. A strength member is located in the center of the slotted core cable structure. That is, in the slotted core construction of optical cable, a profile member is extruded around a central strength member made of metallic or dielectric material. A plurality of slots or grooves which follow a helical or reversing helical path are located on the outer surface of the thermoplastic profile member. One or more optical fibers lay in the slots in a virtual stress-free condition. The optical fibers may be loosely configured, grouped in bundles wrapped with binders, or held in a ribbon matrix.
Finally, in a loose tube or flextube structure, several buffer tubes containing optical fibers are stranded around a central strength member. The buffer tubes are then typically bound together with a separate binder before being enclosed within a common sheath. With respect to identifying and manipulating the optical fibers without disturbing or damaging other optical fibers within a cable, the loose tube or flextube structure offers advantages over the monotube. A single buffer tube may be accessed in the loose tube or flextube structure while the remainder of fibers within other buffer tubes are undisturbed. In contrast, entry into the single central monotube is likely to increase the risk of damaging adjacent fibers because all of the fibers are contained within the single monotube.
Loose tubes include extruded cylindrical tubesxe2x80x94called buffer tubesxe2x80x94which enclose optical fibers in a cable. The optical fibers enclosed within a loose tube may be in the form of single optical fibers, optical-fiber ribbons, or any other configuration of optical fibers, which are simply referred to hereinafter as optical fibers for convenience. The buffer tubes serve many purposes, for example: providing physical protection to the optical fibers; protecting the optical fibers from contaminants; containing water blocking materials; isolation of the optical fibers into groups; strengthening the cable to resist crushing forces; and providing room for optical fibers to move when the cable is bent and when tension is applied to the cable.
In conventional methods, individual loose tubes are first formed, as in a buffering process, and they are then stored as an intermediate product, for example on a plate or drum. A plurality of these loose tubes are then stranded togetherxe2x80x94in a separate stranding process which often takes place in a different locationxe2x80x94to form an optical-fiber cable, or loose tube unit. An example of this type of method is disclosed in U.S. Pat. Nos. 5,938,987 and 4,171,609, wherein the former discloses a method by which individual loose tubes are formed, i.e., a buffering step, and the latter discloses a method by which individual preformed loose tubes are stranded together, i.e., a separate stranding step. However, winding the loose tubes onto a drum or depositing the loose tubes represents additional work outlay and cost. Further cost is associated with storage and transportation of the individual loose tubes to the stranding location.
U.S. Pat. No. 5,283,014 attempts to solve the problems of using separate process lines and/or locations to first form individual loose tubes (buffering) and then strand them together. This patent consecutively disposes buffering and stranding lines so that individual loose tubes are formed, are cooled so as to solidify, and are then stranded together along one process line, thereby avoiding storage and transportation costs involved in other conventional methods. However, the buffering and stranding processes are still separate and, therefore, this process still suffers drawbacks associated with separate buffering and stranding processes, such as high cooling costs and low line speeds as it completely cools the loose tubes before it strands them together. Further, the loose tubes are bound together with a separate binder before being enclosed within a common sheath, thereby adding process time as well as expense.
Flextubes are similar to loose tubes in that they contain a supporting sheath which surrounds optical fibers in a cable. The optical fibers enclosed within a flextube may be in the form of single optical fibers, optical-fiber ribbons, or any other configuration of optical fibers, simply referred to hereinafter as optical fibers for convenience. The supporting sheaths of flextubes serve many purposes, for example: providing physical protection to the optical fibers; protecting the optical fibers from contaminants; containing water blocking materials; isolation of the optical fibers into groups; and strengthening the cable to resist crushing forces.
Although flextubes are similar to loose tubes, they have several differences. In particular, flextubes for a given number of optical fibers have an outside diameter which is smaller than that for a loose tube having the same number of optical fibers. In other words, the supporting sheath of a flextube lies more tightly around the optical fibers than does a buffer tube of a loose tube. That is, the supporting sheath is disposed in contact with the optical fibers so as to surround them in such a manner as to achieve mechanical coupling between the optical fibers. Alternatively, there may be a very small space between the supporting sheath and some of the optical fibers therein. Further, the supporting sheath is more flexible than a buffer tube, and is not necessarily cylindrical as is a buffer tube. That is, the supporting sheath conforms to the optical fibers which it surrounds, whereas a buffer tube is a rigid cylinder having the optical fibers therein. Because of the manner in which flextubes are formed, they are lighter in weight and smaller in size than their loose tube counterparts. Further, because the supporting sheaths are more flexible than buffer tubes, it is easier to accessxe2x80x94without special toolsxe2x80x94the optical fibers within a flextube than it is to access the optical fibers within a loose tube. That is, the supporting sheaths may be easily removed with bare fingers or by simple tube access tools.
Despite the differences between flextubes and loose tubes, flextubes are formed into optical-fiber cables in a similar manner as described above for loose tubes. Accordingly, the heretofore methods of producing flextubes into flextube units or optical cables suffer the same disadvantages noted above for the production of loose tubes into optical cables.
An object of the present invention is to overcome the disadvantages of the related art. In particular, it is an object of the present invention to increase line speed and decrease cost in the production and use of flextube units, or optical cables.
The present invention achieves the above and other objects and advantages by providing a new process, in manufacturing optical cables, called solid-stranding. The process of solid-stranding is a one-step process for extruding multiple loose tubes or flextubes (hereinafter simply referred to as flextubes for convenience) and stranding the tubes in helical or SZ stranded arrangement prior to winding the optical cable on a take-up reel. The method and apparatus of the present invention, for solid-stranding, combine together buffering and stranding operations, as well as perform the stranding operation while the supporting sheaths of the flextubes are still hot so that they adhere together without additional binders. In particular, optical fibers and/or wires are supplied to an extruder which forms supporting sheaths around individual ones or groups of the optical fibers and/or wires. A central element is supplied through the center of the extruder. A rotating pulling device, such as a caterpillar, helically strands the flextubes around the central element as the sheaths cool down. That is, in the new solid-stranding process of the present invention, buffering and stranding operations are performed together without a cooling stage therebetween so that the flextubes contact one another and the central element at a temperature which is between the process temperature and the melting point of the material from which the flextubes are made. The process temperature is that of the material in the extruding die and, typically, is 20-50xc2x0 C. above that material""s melting point, wherein the melting point is typically a range of temperatures over which the material melts. Although the melting point is actually a range of temperatures over which the material melts, the term xe2x80x9cmelting pointxe2x80x9d is used for convenience. Thus, because the flextubes are brought together at a temperature which is between the process temperature and the melting point, they adhere together forming a composite core. The composite core then may be jacketed by passing through another extruder, or may be used without a jacket, thereby forming a flextube unit from which individual flextubes easily may be split.
Because there is no need to completely cool the flextubes before they are stranded, and the flextubes have a relatively thin supporting sheath wall, water cooling of individual flextubes can be replaced by air cooling the composite core. That is, cooling water right after extrusion becomes unnecessary. Therefore, any water swellable elements used in the flextube unit are not damaged. Additionally, air coolingxe2x80x94as opposed to water coolingxe2x80x94the flextubes may help to improve the breakability and split of the supporting sheath, when it is desired to do so. That is, water cooling may undesirably change the mechanical behavior of some polymers, like those based on polypropylene (PP). Such undesirable change in mechanical behavior is eliminated by air cooling the flextubes after they have been stranded together.
Further, solid-strandingxe2x80x94wherein buffering and stranding are performed in the same processxe2x80x94shortens manufacturing time. That is, skipping the cooling stagexe2x80x94wherein individual flextubes are formed, which must then be combined in a stranding phasexe2x80x94increases production speed.
Moreover, solid-stranding forms a composite core which eliminates the need for separate binders to hold the flextubes together. Conventional cable designs produced by separate buffering and stranding processes may use a minimum of one binderxe2x80x94typically a polyester yarnxe2x80x94to secure all the loose tubes or flextubes during the tube stranding step. The binder allows the tubes to remain at desirable locations, such as in helically or SZ-stranded arrangements. However, the binders add costs in manufacturing material and equipment, as well as labor burden in the field to remove the binder prior to accessing the tubes. Because separate binders are not necessary in the solid-stranding process of the present invention, production speed is further increased and, at the same time, production cost is reduced. Further, such results in a lower cost optical cable, the tubes and optical fibers of which can be more easily accessed in the field thereby reducing labor costs involved with use and repair of the optical cable.
The solid-stranded flextubes may be continuously helically stranded in one direction, or may change stranding directions to form an SZ-stranded composite core. Further, the central element may be a water swellable yarn, a ripcord, or itself may be a flextube having optical fibers therein. Alternatively, the central element may be a metallic or other heat conducting member. In this case, the central element can be heated to further promote adhesion between the flextubes stranded around it, as well as to assist in sticking the flextubes to the central member.
Although a single central element is described above, such is not necessary. Instead, a plurality of flextubes may be stranded together without a central element. Further, the flextubes may be stranded together with water blocking elements such as yarns or powders. The flextube unit may then be wrapped in a water blocking tape, surrounded by armoring, and then encased in a sheath having strength members therein.
The individual flextubes which make up the solid-stranded flextube unit have sheaths made of conventional materials such as, for example, plastic material including polyethylene (PE), polybutylene terephthalate (PBT), polypropylene (PP), polyvinyl chloride (PVC), polyamide (PA), and/or ethylene vinyl acetate (EVA), as well as copolymers and/or blends of the above materials.